Americans are getting fatter. A recent Gallup Poll found that nearly 30 percent of adult Americans are considered obese.

Research on the obesity epidemic to date has identified a long list of contributing factors, including ethnicity, income, bigger portion sizes, and fast food, among many others.

But according to a recent report published in the journal CA: A Cancer Journal for Clinicians, scientists should rethink entirely the way obesity research is conducted and what causes of the problem we should focus on.

Many obesity studies looks at certain subgroups of the population. For example, earlier this year we reported on a study that found obesity rates were decreasing among teens from well-educated families, but continuing to rise among poor teens.

This new research found that Americans across all income levels have been gaining weight at similar rates for the past 25 years. That suggest that obesity is driven by environmental factors that affect the entire population, not just particular subgroups.

Researchers dispel popular obesity-related myths

Myth One: Americans are fat because we don't have time to work out

People have blamed obesity on the fact that we are overworked and don't have time to exercise, but actually, over the past half century, there has been a reduction in work hours and an increase in free time. According to the Bureau of Labor Statistics, in 2012, people spent 10 minutes less on work and watched 15 minutes more of TV or napped 10 minutes more than they did in 2003.

Myth Two: Americans are fat because we get less exercise than we have in the past

We often hear that advances in technology, and computers in particular, have contributed to more sedentary lifestyles. But, as it turns out, Americans are actually exercising more today than in the past. In 2001, 46 percent of Americans reported at least 30 minutes of moderate activity on 5 or more days per week. The number increased to 51 percent in 2009, according to data from the Centers for Disease Control and Prevention.

Myth Three: Americans are fat because they don't have access to grocery stores

The term "food desert" has been popularized over the past few years to describe an urban location that doesn't have great access to grocery stores that sell fresh produce. These communities tend to have higher rates of obesity — and naturally people have drawn a link.

But several recent studies have found there isn't a significant relationship between distance from food stores and obesity. And, surprisingly, fruit and veggie consumption in the U.S. has actually increased over the past few years. Americans are eating more broccoli, cauliflower, tomatoes, onions, apples, bananas, and grapes than ever before.

Why, then, are we so fat?

Instead of the myths about food and obesity above, the report's authors suggest the cause of our unhealthy weight could be more general: the overall cost and availability of food in the United States.

"The high cost of healthy food may not be the problem as far as obesity is concerned, rather it is the excess availability and affordability of all types of food," said Roland Sturm, lead author of the report and a senior economist at RAND, a nonprofit research organization in a statement.

Food in the U.S. is currently at the cheapest level in history when measured as a fraction of disposable income. For example, in the 1950s, Americans spent one-fifth of their disposable income on food. Nowadays, people in the U.S. are spending an average of one-tenth of disposable income on food.

This means less money buys more calories for people at every economic level and that all Americans are exposed to an environment that tends towards obesity. This is true at every level of wealth.